#include<bits/stdc++.h>
using namespace std;
inline int read()
{
	int x=0,f=1;
	char ch=getchar();
	while(ch<48||ch>57){if(ch=='-')f=-1;ch=getchar();}
	while(ch>=48&&ch<=57)x=(x<<1)+(x<<3)+(ch&15),ch=getchar();
	return x*f;
}
inline void write(int x)
{
	if(x<0)putchar('-'),x=-x;
	if(x>9)write(x/10);
	putchar(x%10+48);
	return;
}
inline void openfile()
{
	freopen("prize.in","r",stdin);
	freopen("prize.out","w",stdout);
}
constexpr int M=1e5+4;
int n,m,num[M];
multiset<int>s;
inline bool check(int x)
{
	s.clear();
	int np=num[1]+num[x];
    for(int i=2;i<=n;++i)
	{
        if(i==1||i==x)continue;
        s.insert(num[i]);
    }
    int rk=0;
    while(!s.empty())
	{
        int t=*s.rbegin();
        auto it=s.end();
        s.erase(--it);
        it=s.upper_bound(np-t);
        if(it==s.end())break;
        ++rk;
        s.erase(it);
    }
    return rk<m;
}
int main()
{
	openfile();
	n=read(),m=read();
	for(int i=1;i<=n;++i)num[i]=read();
	sort(num+2,num+n+1);
	int l=1,r=n+1,ans=n+1;
	while(l+1<r)
	{
		int mid=l+r>>1;
		if(check(mid))ans=mid,r=mid;
		else l=mid;
	}
	if(r==n+1)puts("-1");
	else write(num[ans]);
	return 0;
}